Lymphocytes are important effectors of tissue destructive reactions typified by allograft rejection, tumor immunity, and delayed hypersensitivity states. These are complex reactions, and it is not yet clear how effector cells cause actual cell lysis. Our studies, conducted in in vitro systems, are directed at elucidating the molecular mechanism(s) of cell lysis. We have identified an inducible system of cell-lytic molecules released by lectin activated human and murine T-enriched and perhaps NK cells in vitro. This lytic system consists of lymphotoxins (LT), stabilizing factor(s), and antigen binding receptor(s) (R), which are not classical Ig molecules. Human forms express alloantigens and antigens associated with the Clq component of complement. Activated effector cells first release these molecules associated together in complexes (Cx), which are unstable and rapidly dissociate into components and intermediate forms. The smaller components (LT) are only weakly cell-lytic in vitro, however, the larger forms can cause "nonspecific" lysis of certain cells. Moreover, alloimmune T-cell -enriched cell populations can release Cx + R forms which can specifically bind and cause specific target cell destruction. Employing biochemical and immunochemical techniques, we will continue to identify, isolate, and characterize the components of this system from human and murine T and NK effector cells.